There’s a gap between the song you intend to write and the song you actually write.
Any dissatisfaction you feel at the end of a session comes from this gap.
You did the best you could at the time and it wasn’t enough to connect the desired vision to the actual outcome. It’s a learning and development process over time (mixed in with some luck) that brings them closer together.
While it’s almost impossible to streamline or hurry the process along, the simple next step on the journey is yours for the taking.